1. Field of the Invention
This invention relates generally to holography and in particular to holograms, holographic optical elements, and other wavefront-modulating structures capable of being addressed and controlled electrically. This invention relates also to methods of fabricating such holograms, holographic optical elements, and wavefront-modulating structures.
2. Description of Related Art
For nearly three decades, there has been a need for electrically controllable holograms, especially holograms that can be controlled at high speeds. A major objective of holography research and development has been realization of apparatus for three-dimensional dynamic displays. Such displays would find use in holographic stereoscopic motion pictures or television and many other display applications. In an article published in 1965, Leith et al. "Requirements for a Wavefront Reconstruction Television Facsimile System" J. SMPTE Vol. 74 (1965), the requirements for dynamic holographic displays were enunciated. In 1969, referring to the information content requirements of holograms, W. E. Kock stated that" . . . the outlook for using holograms in television is very bleak." (W. E. Kock "Lasers and Holography" Doubleday & Company 1969, reprinted by Dover Publications 1981).
More recently a real-time dynamic holographic display was constructed and operated (St. Hilaire et al. "Real-time Holographic Display: Improvements Using a Multichannel Acousto-optic Modulator and Holographic Optical Elements" Practical Holography V, SPIE Vol. 1461, (1991) Pages 254-261).
A number of different technologies have been applied to the problem of dynamic holographic displays. The field was reviewed by the present inventor (W. P. Parker "Output Devices for Dynamic Electronic Holography" Master's Thesis, Massachusetts Institute of Technology Media Laboratory, January 1989). (See also W. P. Parker "Output Devices for Electro-Holography" SPIE Proceedings Vol. 1667, Practical Holography VI, San Jose, Calif. 1992.) That research resulted in descriptions of numerous technologies that might be used in output apparatus for holography. Such apparatus has been denoted by several names including Electrically Addressed Hologram (EAH), Dynamic Electronic Hologram (DEH), and most recently the Electronic Hologram or Electrohologram (EH). An addressable electrohologram may be abbreviated AEH. The latter terminology will be adopted for the invention of the present application. The terms "addressable electrohologram" and "electrically addressable hologram"are used interchangeably in this specification and in the appended claims. These terms are also used herein to denote any wavefront-modulating device controlled by electrical means, whether the device carries a holographic image or a non-image wavefront-modulating structures are equivalent to holograms. For example a linear diffraction grating of uniform pitch acts as a hologram of a plane wave.
Dynamic holographic displays use primarily image-bearing holograms. Besides dynamic holographic displays, electroholograms are useful for other non-image bearing applications examples are beam steering modifying, or modulating tasks for holographic or non-holographic optical memory, for optical computers, for optical correlators for scanning (such as bar code scanners), for bariable-parameter holographic optical elements (HOE's)(such as variable-power or zoom lens systems, or variable spectroscopic gratings), for spatial light modulating devices (such as page composers and light valves), for visible laser diode arrays, and for integrated optic applications. Many of these applications required a device with high space-bandwidth product, ranging from 10.sup.3 to 10.sup.10 or higher.
U.S. Statutory Invention Registration H738 (McManus et al. 1990) discloses switching of beams among a number of holograms for reconfigurable optical interconnections. This device utilizes an array of optical switches which direct a set of combination of a twisted-nematic liquid crystal cell and a polarizing beam splitter. The holograms themselves are not switched.
U.S. Pat. No. 4,850,682 (Gerritsen et al., 1991) discloses diffraction grating structures in which at least one topographic-relief diffraction grating is placed in contact with a liquid crystal material having substanially the same refractive index as the grating when the liquid crystal is in an inactivated state, but having a substantially different refractive index when the liquid crystal is in an activated state. When the liquid crystal is in an inactivated state, light incident on the structure passes through the structure and exits in a direction subtantially unchanged from its incident direction. When the liquid crystal is activated with an electric field such incident light is diffracted in a new direction. There is a direct electrical connection to each element or set of elements used for activating the liquid crystal.
U.S. Pat. No. 5,096,282 (Margerum et al., 1992) discloses polymer dispersed liquid crystal (PDLC) film devices including special types forming gratings and holograms. The grating and hologram films are obtained by periodically varying the conditions of polymerization over the film to produce a corresponding periodic spatial variation in liquid crystal bubble size. Since there are correlations between the liquid crystal bubble size and the resulting film's threshold and operating voltages for optical transmission,the grating or hologram films are capable of time modulation by applied voltages. The operation of such films depends on light scattering from the liquid crystal bubbles.
U.S. Pat. No. 5,111,313 (Shires, 1992) shows an electronically modulated holographic autosteroescope utilizing a cylindrical holographic optical element which is spun about its axis by a motor. The holographic optical element comprise two different hologram directs each raster scan out of the cylinder at a specified angle. In this type of display, the holograms are not tatic, as they are spinning, but the modulation that produces a display of dynamic information is achieved by modulating the laser beam light source.
U.S. Pat. No. 5,151,724 (Kikinis, 1992) and U.S. Pat No. 5,266,531 (1993) disclose a dynamic holographic display in which the hologram itself is modulated electro-mechanically. It has an array of reflective surfaces formed on cantilevered structures substantially parallel to the surface of a substrate, such as a silicon wafer. Electrical currents are used to position the individual reflective surfaces in the array so the topography forms a hologram, and reflected light forms a holographic image. In fabricating such a display, the lithographic techniques used must of course have sufficient resolution to form cantilevered structures spaced at a punch appropriate to the desired hologram.
U.S. Pat. No. 5,172,251 (Benton st al., 1992) discloses a three-dimensional display system using an acousto-optic modulator, various lenses, and horizontal and vertical scanners. A data processing system sends signals encoding a diffraction pattern to the acousto-optic modulator which generates a three-dimensional image such as a holographic image.
U.S. Pat. No. 5,191,449 (Newswanger, 1993) discloses an animated holographic stereogram display in which a hologram contains a plurality of stereographic image pairs, and a user views the hologram from a fixed relative position. A plurality of light sources are spatially dispersed and operated in appropriate time sequencing to animate the image pairs. This is an example of several dynamic holographic displays in which electrical control is used to control one or more light sources, while the holograms themselves are static.
U.S. Pat. No. 5,198,912 (Ingwall et al., 1993) discloses a volume phase hologram with liquid crystal material filling microvoids between the holographic interference fringes formed in a material having a different index of refraction. The diffraction efficiency of such composite volume phase holograms may be varied, e.g. by application of an electric field.
While the holographic displays described above are useful for many of the desired purposes, there exists a need for electrically addressable holographic elements specifically aimed at high frequency operation for real-time applications, and capable of being fabricated by methods compatible with semiconductor wafer processing, for eventual integration with VLSI or ULSI semiconductor electronics such as the wafer-scale electrohologram envisioned in the present inventor's 1989 thesis mentioned above. There is also a need for addressable wavefront-modulating devices that are substantially planar and very thin. Thin structures are especially needed so that combinations in stacked structures will have practical total thickness. There is especially a need for electrically addressable wavefront-modulating devices that do not require direct electrical connection to each fringe or diffracting element, that do not require electrical continuity among the fringes or diffracting elements of a pattern, and that operate at high frequencies.
In manufacture of ordinary conventional liquid crystal displays such as alphanumeric displays, the elements forming alphanumeric characters are made with dimensions of elements and their spacings (typically with spatial frequencies much greater than 100 cycles per millimeter) that enable them to be easily visible by the display user. The individual segments or pixels of such conventional liquid crystal displays are also individually addressed, each being connected to an electrical drive line that can be switched to turn the segment or pixel on or off. These may be described as element-addressed or matrix-addressed. In contrast to such liquid crystal displays, an addressable electrohologram requires modulating elements at considerably higher spatial frequencies, typically more than 100 cycles per millimeter, to produce the desired diffraction effects on incident light. Since holograms often consist of many small elements without continuity among the elements, an addressable electrohologram can also benefit from area addressing or field addressing instead of the element addressing or matrix addressing of conventional liquid crystal displays. Thus a device capable of being addressed over an area or field containing many discontinues small diffracting elements is especially useful.